Motor-driven power steering system

Abstract

A motor-driven power steering system includes a power assisting motor; a motor current sensor, a sample-hold circuit that has sampling section and a holding section, a timing signal generating circuit that generates a timing signal to activate either the sampling section or the holding section and an alarming unit that generates an abnormality signal if the timing signal is not detected for a prescribed period.

1. A motor-driven power steering system comprising:
 a power assisting motor; 
 means for detecting motor current value; 
 means for sampling and holding the detected motor current value at prescribed time intervals; 
 means for generating a timing signal to activate the means for sampling and holding to continuously hold the detected motor current; and 
 means for generating an abnormality signal if the timing signal is not detected for a prescribed period. 
 
   
   
     2. The motor-driven power steering system as claimed in  claim 1 , wherein the means for sampling and holding is activated if the timing signal is not detected for the prescribed period. 
   
   
     3. The motor-driven power steering system as claimed in  claim 2 , further comprising means for calculating command current value to be supplied to the power assisting motor, wherein the maximum of the command current value is set to the detected motor current value if the timing signal is not detected for the prescribed period. 
   
   
     4. The motor driven power steering system as claimed in  claim 3 , further comprising means for gradually decreasing the command current value. 
   
   
     5. A motor-driven power steering system comprising:
 a power assisting motor; 
 a current sensor that detects motor current value; a timing signal generating circuit that generates bi-level timing signals at prescribed intervals; 
 a sample-hold circuit for sampling and holding the detected motor current value at the prescribed time intervals set by said timing signal generating circuit; and 
 an abnormality detecting circuit that makes said sample-hold circuit keep holding the last sampled value if the timing signal is not detected for a prescribed period. 
 
   
   
     6. The motor-driven power steering system as claimed in  claim 5 , wherein:
 said abnormality detecting circuit comprises a counter circuit connected to said timing signal generating circuit; and 
 the prescribed period is generated when said counter circuit counts a prescribed number. 
 
   
   
     7. The motor-driven power steering system as claimed in  claim 6 , further comprising a torque sensor that detects steering torque and an assist current calculating circuit that calculates a command current value to be applied to said motor from both the current value sent from the sample-hold circuit and the steering torque sent from the torque sensor to control the motor, wherein the maximum of the command current value is set to the detected motor current value if the timing signal is not detected for the prescribed period. 
   
   
     8. The motor driven power steering system as claimed in  claim 7 , further comprising means for gradually lowering the command current value. 
   
   
     9. The motor driven power steering system as claimed in  claim 1 , wherein said means for generating a timing signal comprises a microcomputer. 
   
   
     10. The motor driven power steering system as claimed in  claim 9 , wherein said means for generating an abnormality signal is disposed out of said microcomputer. 
   
   
     11. The motor driven power steering system as claimed in  claim 5 , wherein said timing signal generating circuit comprises a microcomputer. 
   
   
     12. The motor driven power steering system as claimed in  claim 11 , wherein said abnormality detecting circuit is disposed out of said microcomputer.